Thickness shear mode quartz resonator sensors (also interchangeably called quartz resonator transducers) have been used successfully in the down-hole environment of oil and gas wells for several decades and are still an accurate means of determining bottom-hole pressure and temperature. Quartz resonator pressure and temperature sensors typically have a crystal resonator located inside a housing exposed to ambient bottom-hole fluid pressure and temperature. Electrodes on the resonator element coupled to a high frequency power source drive the resonator and result in shear deformation of the crystal resonator. The electrodes also detect the resonator response to at least one of pressure and temperature and are electrically coupled to conductors extending to associated power and processing electronics isolated from the ambient environment. Ambient pressure and temperature are transmitted to the resonator, via a substantially incompressible fluid within the housing, and changes in the resonator frequency response are sensed and used to determine the pressure and/or temperature and interpret changes in same. For example, a quartz resonator sensor, as disclosed in U.S. Pat. Nos. 3,561,832 and 3,617,780, includes a cylindrical design with the resonator formed in a unitary fashion in a single piece of quartz. End caps of quartz are attached to close the structure.
Generally, a thickness shear mode quartz resonator sensor assembly may include a first sensor in the form of a primarily pressure sensitive quartz crystal resonator exposed to ambient pressure and temperature, a second sensor in the form of a temperature sensitive quartz crystal resonator exposed only to ambient temperature, a third reference crystal in the form of quartz crystal resonator exposed only to ambient temperature, and supporting electronics. The first sensor changes frequency in response to changes in applied external pressure and temperature with a major response component being related to pressure changes, while the output frequency of the second sensor is used to temperature compensate temperature-induced frequency excursions in the first sensor. The reference crystal, if used, generates a reference signal, which is only slightly temperature-dependent, against or relative to which the pressure- and temperature-induced frequency changes in the first sensor and the temperature-induced frequency changes in the second sensor can be compared. Means for such comparison as known in the art include frequency mixing or using the reference frequency to count the signals for the first and second sensors.
Prior art devices of the type referenced above including one or more thickness shear mode quartz resonator sensors exhibit a high amount of accuracy even when implemented in an environment such as a down-hole environment exhibiting high pressures and temperatures. However, such thickness shear mode quartz resonator sensors may be relatively expensive to fabricate, as each sensor must be individually manufactured. These relatively expensive quartz resonator sensors may not be economically practical for implementation in applications that would benefit from their relatively higher accuracy and ability to operate in a relatively wider range of temperatures and pressures as compared to other less expensive, less accurate and less robust sensors such as strain or piezoresistive gages.